Santorum, after storming into the state after a three-state sweep Feb. 7, slipped in the closing days by letting go the populist economic message that helped him emerge as the chief threat to Romney.

Romney fought hard against the challenge from Santorum, as he did in quelling the advance of Newt Gingrich in Florida.

The front-runner seized on comments Santorum made during an Arizona debate last week, when the former Pennsylvania senator said he had voted for legislation he disagreed with in order to support his party. The comments allowed Romney to cast himself as an outsider from unpopular Washington and cast Santorum as a Beltway creature.

Romney also pointed to votes by Santorum, a hero to anti-abortion activists, for legislation that helped finance abortion provider Planned Parenthood. And he slammed Santorum's endorsement of Arlen Specter, a former Pennsylvania Republican-turned-Democratic senator who supported abortion rights.

By attacking Santorum, Romney took advantage of how little voters knew about the candidate who campaigned in obscurity throughout last year, only to emerge as the surprise winner in the Jan. 3 Iowa caucuses.

"Romney nailed Santorum upside the head with his record on spending and his Washington connections. I think that resonated," said Jeff Timmer, a Michigan GOP campaign strategist and former Michigan Republican Party director. "By primary day he wasn't the shiny object anymore. The new car smell was gone."

A political action committee that supports Romney, Restore Our Future, came to his aid in Michigan, just as it did in Florida and Iowa last month. The group spent more than $1 million in the final week of the Michigan campaign on TV ads attacking Santorum. They helped give Romney a better than 2-to-1 edge in ad spending over Santorum and a group that supports his candidacy.

Romney built an early edge in absentee votes cast before Tuesday, the product of months of planning to compete again in the primary he won in his unsuccessful bid for the 2008 nomination. As much as one-quarter of Tuesday's votes were expected to have been absentee, party officials said.

Romney also made a direct appeal to Michigan's struggling economy by proposing measures that he argued could help the struggling automotive industry recover over the long term.

Santorum challenged Romney with a fearless assault on the native son's home turf, and got help from outside groups, evangelical conservatives and tea party supporters, the same coalition that helped him break through in Iowa last month.

"A month ago they didn't know who we are, but they do now," Santorum told supporters in Grand Rapids on Tuesday.

Santorum's crowds in the closing days of the campaign were large and loud. Santorum fueled their zeal with a full-throated attack on President Barack Obama, painting himself as the philosophical opposite to the Democrat.

"Santorum really picked a smart fight with the president, a cultural fight, over the weekend," said Stu Sandler, a veteran Republican campaign operative in Michigan. "You can argue about the effects overall, but when the media and the president jump on a GOP candidate, the base will react."

But it also threw Santorum off message.

So did Romney's attacks on Santorum's conservative bona fides. Santorum spent the first 15 minutes of a speech in Flint Sunday night blasting Romney as a false conservative. "To be attacked on television as someone who is not an authentic conservative by a Massachusetts governor is a joke," Santorum said Monday in Lansing.

In the closing days, Santorum criticized Romney's tax proposal as weak and promoted his own plan to spark manufacturing.

But he tripped again Monday, when it was revealed that Santorum's campaign was courting Democratic votes via automated telephone calls. Santorum defended the calls, saying, "We're going to get voters that we need to be able to win this election."

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